Try to count the number two independent IRC files by adding a "reverse" in one and "forward" in the second ....

example:

IRC=(calcfc, maxpoints=..., stepsize=...., forward)

IRC=(calcfc, maxpoints=..., stepsize=....,reverse)

Another possibility: reoptymalization of TS structure by u PCM model using structure from gas phase led to structure that is not in the path of the reaction. In this case, try to locate the TS again in a polar medium (eg. QST2 method).

Dear Radomir, 

actually I have tried to conduct IRC calculations with two independent IRC files (adding reverse and forward options), however these two calculations generated the same reaction intermediate, i.e., the same path. What I have tried was decrease the stepsize, hoping that it was a numerical problem, but the problem still remains unsolved. 

I don't know if reoptimize the TS could be a good option, because it is well characterized and the model is reasonable in chemical terms. 

Anyway, I appreciate your help.

Just to illustrate, This is the file showing the IRC calculated in aqueous phase. In this situation I obtained two different paths but the points accumulates in the same region, not showing variation in the stepsize. 

It really seems like your TS is accurate (the energy decreases in both directions).

Some thoughts:

- the PES around the TS doesn't seem very flat, so you could actually try decreasing the stepsize.

-what is the end message at the end of your calculation? (I assume you are using Gaussian)

-have you tried computing the frequencies with PCM and see if some other imaginary frequencies appear besides the one connecting reactants to products? These might have some influence on your IRC calculation.

-Could you also show us the graph of gradients along the reaction coordinate? These are given automatically by Gaussview (again, I'm assuming you are using Gaussian).

-what do you mean by "generated the same intermediate"? I see from the attached graph that the energy decreases in both directions from your transition state (irc point 0)

Dear Mihai, thank you for your response.

Actually I have different IRC calculations for different transition states, generating these two problems

#1 IRC calculations leading to the same reaction intermediates in both paths. (File IRC-for-rev-grad-TS-1-normal-mechanism-PCM-UAHF).

#2 IRC points accumulating in the end of IRC graph. (File IRC-for-rev-grad-TS-1-concerted-mechanism-PCM-UAHF)

These TSs were fully unconstrained optimized using PCM model with the UAHF cavity. Frequency calculations, which confirms the TS nature (single imaginary frequency) and IRC calculations were also conducted within the continuum approach.

"generated the same intermediate" means that two different IRC calculations (forward and reverse) generated the same chemical structure for both sides.

The file attached shows the IRC profile and the gradients along the reaction coordinates.

The calculation where the points accumulate at the end of IRC curve have not finished, for this reason there was no error message. 

Ok. So my opinions related to these matters:

- for the concerted mechanism, based on the gradient graph, it seems like you located your TS. I usually saw the accumulation that you talk about in calculations which ended with an error. Try restarting the calculation or decreasing the step size.

- for the "normal mechanism", I can see that the transition state is at a maximum on the gradient graph, which is completely wrong. For the TS: you should have a minimum on the gradient graph and a maximum on the relative energy graph, so the TS is incorrect.

-if I understood correctly, you said these are results for the "reverse" IRC, right? Well, to be best that I can judge from all the graphs that you showed me, the IRC is followed in both directions (aka both in the reverse and forward directions, because the TS should always be at point 0). Why is that?

offo!! even I have the same problem... Did you find any solutions Lucas??

Dear Mihai, thank you for your suggestions. I spend the weekend analizing my structures and your intuition was right, the IRC for normal mechanism is wrong. The TS is right but the IRC was conducted assuming the gas phase as the surroundings, and the TS strucutre was obtained within the continuum media. So I remake the calculations using now the IRC in aqueous phase:

# irc=(calcfc,maxpoints=60,forward,stepsize=20,maxcyc=200) scf=(maxcycle=200) b3lyp/gen gfinput pseudo=read scrf=(pcm,read)

The problems with points accumulating in the extremity of the curve has appeared again, but the calculation is still running. I will wait until it finishes to comment about the results. 

Once again thanks for the helpful assistance

Dear Prerna, unfortunately i did not find any solution until now, but I am working hard to understanding what is happening with IRC calculations using PCM. I saw a comment in Joaquim Barroso's Blog about an incompatibility between IRC and PCM in Gaussian 03 (though I'm using G09), but he says that it is not true for Gaussian 09, where there is a new algorithm for PCM calculations.

Blog link:

http://joaquinbarroso.com/2009/09/07/pcmg03/#comment-251

I am looking for similar problems in CCL, but I did not find anything similar until now. 

I also encountered similar problem. Did you already find a solution to this problem?

For the first time I've just found two separate IRCs (one forward, one reverse) using the same TS have produced the identical IRC energy descent. Any idea what's going on?